Drills having the point angle set to 120° to 140° are provided as a standard model.
Such drills, however, have a problem of low accuracy of the hole position when the drill bites a non-flat surface such as inclined surface in processing the non-flat surface. Thus, drills suitable for processing a non-flat surface have been developed.
Japanese Patent Laying-Open No. 2004-141970 (PTD 1) discloses a drill having a point angle of 170° to 180° as a drill for drilling a hole in an inclined surface.
Japanese Patent Laying-Open No. 2009-56534 (PTD 2) discloses a drill functioning as a counterbore drill for drilling a guide hole or performing boring as preprocessing before drilling a hole in an inclined surface. The counterbore drill has a point angle of 170° to 190°, and includes two secondary cutting edges extending outward from the chisel through thinning, a primary cutting edge in a concave shape extending outward from each secondary cutting edge, and an outer cutting edge extending from each primary cutting edge to a leading edge and receding in the rotational direction of the drill. The outer cutting edge has a receding angle of −1° to −20°.
A drilling operation in a workpiece by a drill of a standard model forms a blind hole with a cone-shaped bottom. For certain applications, however, blind holes with a flat bottom are desired. For such applications, Japanese National Patent Publication No. 2008-500190 (PTD 3) discloses a drill having a point angle of 180°.